The Syrian hamster has a remarkable sex limited serum protein, expressed as a major protein in females (therefore called Female Protein) and testosterone suppressed in males. Female Protein (FP) is a homolog of two human pentraxins, C-reactive protein (CRP) and amyloid P component (AP) as shown by similar structure (pentamer of noncovalently assembled monomer subunits) and amino acid sequence. (FP is 69% identical to human AP sequence.) Furthermore, FP shares functional properties with both human pentraxins such as Ca dependent phosphorylcholine binding, complement fixation, acute phase responsiveness (characteristics of CRP) and also is a constituent of amyloid (characteristic of AP). In addition to Syrian ha;mster FP, another FP now has been characterized in the Armenian hamster. Both Syrian and Armenian FP have been shown to be capable of an efficient reversible dissociation from pentamer to monomer subunits in vitro. Furthermore, a hybrid FP pentamer was constructed by reassociating mistures of monomer FP subunits of Syrian and Armenian FP. Reversible dissociation may play an important role in pentraxin function. An FP homolog has not yet been found in the Chinese hamster, even after administration of various steroids. However, these hamsters manifest unusual responses to steroids including DES induced icterus from hepatotoxicity and more recently a medroxyprogesterone induced diabetes insipitus of apparent hypothalamic origin. such response to steroid injection are without precedence in other mammals and possible could be related to absence of FP.